1. Field of the Invention
The present invention relates to a Faraday rotator constituting an optical isolator and an optical circulator used for optical-fiber communication, optical recording, optical measuring system, etc.
2. Description of the Related Art
In optical-fiber communications systems using semiconductor lasers as light sources, especially in optical systems by the high-speed digital transmission and the analog direct modulation method, when reflected light from optical connector junctions, optical circuit components, etc. used in optical-fiber circuits returns toward the semiconductor lasers or optical amplifiers, frequency characteristics are degraded and noises are generated, giving difficulty to the high quality transmission. Optical isolators are used to eliminate the reflected light.
As shown in FIG. 10, a conventional optical isolator is composed of a polarizer 6 and an analyzer 5 both adapted to allow only light with a specific plane of polarization to pass therethrough, a Faraday rotator 4 comprising a substrate (transparent) 1′ and a laminate film 3 formed thereon and adapted to rotate the plane of polarization of light by 45 degrees, and a permanent magnet (not shown) for applying a magnetic field to the Faraday rotator. The performance of the optical isolator depends primarily on the Faraday rotator 4 among the constituents thereof. It is important for the Faraday rotator to have a small length of an element required for rotating the plane of polarization by 45 degrees and a high light transmittance.
Hitherto, a bulk single crystal (thickness of about 2 mm) of yttrium iron garnet (YIG) and a thick film single crystal (thickness of several hundred μm) of bismuth-substituted rare-earth iron garnet (BiYIG), in which bismuth having a large magneto-optical performance index substitutes for part of yttrium, have been used as the Faraday rotator. Recently, a BiYIG thick film single crystal advantageous in downsizing the optical isolator is often used. This thick film single crystal is produced by a liquid phase epitaxial growth method, in which many production parameters must be precisely controlled to realize a stable epitaxial growth, making it difficult to grow a homogeneous single crystal over a large area at a high yield. Furthermore, since it takes 20 hours or more to grow the crystal, and since an expensive non-magnetic gadolinium gallium garnet (GGG) single crystal substrate is required as the substrate, the cost reduction has been hindered.
Under these circumstances, in order to solve the problems of magneto-optical members produced by the aforementioned liquid phase epitaxial growth method, the inventors of the present invention have suggested a magneto-optical member (Faraday rotator) made of one-dimensional magneto-photonic crystal that brings about the enhancement of a magneto-optical effect due to the localization of light. The magneto-optical member is a polycrystal with a thickness of several μm, but can achieve a large Faraday rotation angle.
The aforementioned one-dimensional magnetic photonic crystal is described in detail in Japanese Patent Application No. 11-283512 filed by the inventors of the present invention, and in Journal of Magnetics Society of Japan 23, 1861–1866 (1999). The one-dimensional megneto-photonic crystal is configured such that magnetic layers and dielectric layers, each layer having an irregular thickness, are formed into a multilayer film, or configured so as to comprise two dielectric multilayer films made of dielectric members having regular thicknesses and alternately laminated, and an irregular layer (defective layer) made of magnetic member.
In particular, the latter has the same structure as a long-known Fabry-Perot resonator structure, and has been known to achieve a large enhancement with a film configuration that can be manufactured easily. While (Ta2O5/SiO2)-system is generally used as the dielectric member constituting a dielectric multilayer film to serve as a reflecting mirror for the Fabry-Perot resonator, (Si/SiO2)-system, which can achieve a large Faraday rotation angle with a smaller number of layers compared with the (Ta2O5/SiO2)-system, has also been suggested. The film thickness of each dielectric member must be designed such that the optical length (optical path length×refractive index) is equal to λ/4 (λ: the wave length of light). The optical length of an irregular layer (defective layer) made of a magnetic member to localize light is generally set to mλ/2 (m: an integer).
However, it is known that the Faraday rotator with the Fabry-Perot resonator structure has a trade-off relationship between the Faraday rotation angle and the light transmittance, and that when the Faraday rotation angle is increased to 45 degrees (or −45 degrees), which is necessary for the optical isolator, the transmittance is reduced to about 50%. 50% of light, that is not transmitted, is reflected at the multilayer film and returns toward the light source, which is very disadvantageous because an optical isolator is used to block returning light. Furthermore, the reduction of the amount of outgoing light leads to the reduction of the light transmission distance, making it difficult to build an optical transmission system.
The inventors of the present invention conducted researches for improvement of transmittance by changing film configurations, and it was found out that a configuration, in which two Fabry-Perot resonators were jointed so as to sandwich therebetween a dielectric thin film having an optical length of λ/4+mλ/2 (m: 0 or a positive integer) (hereafter referred to as D. H. W. (Double Half Wave) structure), is effective in improving the transmittance. As is disclosed in detail in Japanese Patent Application No. 2000-274936, when the film configuration is (Ta2O5/SiO2)8/BiYIG/(SiO2/Ta2O5)8/SiO2/(Ta2O5/SiO2)8/BiYIG/(SiO2/Ta2O5)8, the transmittance and the respectively, at a Faraday rotation angle of 45 degrees, which causes no practical problems.
Although compatibility between the transmittance and the Faraday rotation angle can be achieved by adopting the D. H. W. structure, the D. H. W. structure comprises as many as 67 lamination layers and also has 2 magneto-optical thin films (BiYIG) which need a heat treatment, whereby the process for manufacturing the laminate film is complicated putting limitation to a reduction in the manufacturing cost. Furthermore, there is a problem that when an element is manufactured with an optical length differing from a designed value, reflected light to return toward the light source is generated.